Quad Relay Board

This is a four relay driver circuit. The relays can be triggered using a low current/low voltage source (as little as 3.85V@0.93mA). The version shown here uses 12V SPDT relays but other voltages as well as SPST relays could be used as needed as long as the limits of the IC are respected (<50V <.5A per channel) . If relays other than 12 volts are used the input voltage must be adjusted to correspond and the four LED current limiting resistors should be changed to an appropriate value (see Table 1 for resistor values for common voltages). This circuit uses only 4 of the 7 available channels on the ULN2003A darlington array so three more relays could be easily added. Remember to tie the grounds together if the triggering device is using a different power supply then the one powering the relays. With the parts specified these handy little boards can be built for less than $10 each.

Part #	Part Name	Jameco #
IC1	ULN2003A Darlington Array	34278
RL1..RL4	12V SPDT Relay	172937
D1..D4	Green (3mm) LED	253681
R1..R4	390 ohm 1/4W Resistor	30963
CN1..CN4	3 pin Terminal Block	152354
CN5	2 pin Terminal Block	152346
CN6	.100" Straight Male Header	103350

Table 1. Resistor Values

Voltage	Resistor (ohms)
24	1K
12	390
9	270
5	100